Temperature measuring device utilizing birefringence in photoelectric element

ABSTRACT

A temperature measuring device intended to use the photoelastic effect of a transparent element. The present invention employs as the temperature sensing element a thermal expansion photoelastic cell comprising a photoelastic element and a stress generating element which are closely contacted with each other for yielding stress of anisotropy in the photoelastic element, which changes in response to changing ambient temperature, as the stress generating element is quite remarkably different in thermal expansion coefficient from the photoelastic element. An element is further provided to detect phase difference between two orthogonal light components passed through the photoelastic element which are one polarized component in a stress direction and the other component polarized in a direction perpendicular to the above stress direction when linearly polarized light is passed through the photoelastic element of the thermal expansion photoelastic cell. The detected phase difference is converted into a temperature, which is then displayed on a display device.

FIELD OF THE INVENTION

The present invention relates to an optical temperature measuring deviceand more particularly, a temperature measuring device capable ofmeasuring temperature by using photoelastic effect.

PRIOR ART

Various temperature measuring devices have been well known. The mostpopular one uses a thermocouple as its sensor. The temperature measuringdevice using a thermocouple is used in those cases where temperature ata remote place must be measured. The thermocouple is low in cost andeasy to handle, and moreover can make high accuracy temperaturemeasurements. It is, however, interfered by electromagnetic wave becausethermoelectro-motive force generated between two different metals isvery small. Therefore, the temperature measuring device which uses thethermocouple cannot be used in high electric or magnetic fields, forexample.

The conventional temperature measuring device which can be used in highelectric or magnetic fields measure temperature by using a change of anenergy gap of a semiconductor. If the energy gap changes, the edge ofthe absorption spectrum changes. This change can be detected as thechange of transmission of LE light which has peak spectrum near the edgeof the absorption spectrum. This temperature measuring device uses lightas signals transmission media. This temperature measuring device can beused in high electric or magnetic fields. However, the temperaturemeasuring device is extremely low in sensitivity because the change ofthe energy gap of the semiconductor depending on temperature is small.It is therefore impossible for this temperature measuring device tomeasure a slight change of temperature.

As described above, the conventional optical temperature measuringdevices could not measure temperature with high accuracy because theywere low in temperature sensitivity.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide an opticaltemperature measuring device which is simple in construction and capableof measuring temperature with high accuracy even in high electric ormagnetic field.

A temperature measuring device according to the present inventioncomprises a thermal expansion photoelastic cell including a photoelasticelement and a stress generating means which are closely contacted witheach other for yielding anisotropic stress in a photoelastic element inresponse to ambient temperature because the stress generating means isquite different in thermal expansion coefficient from the photoelasticelement; a means for making linearly polarized light which is enteredinto the photoelastic element of the thermal expansion photoelasticcell; a means for detecting phase difference between two polarized lightcomponents passed through the photoelastic element, which are, forexample, one component polarized in a stress direction or largest stressdirection, and the other one polarized in a direction perpendicular tothe above stress direction thereof; and a means for converting thedetected phase difference into a temperature and displaying theconverted temperature.

When the temperature measuring device is arranged as described above,stress in the photoelastic element changes according to changing ambienttemperature. Stress thus generated in the photoelastic element alsochanges accordingly. This stress has anisotropy. When anisotropic stressis yielded in the photoelastic element, the photoelastic element hasbirefringence (or anisotropy of refraction) depending upon the stress.The degree of this birefringence in the photoelastic element can bedetected phase difference between two orthogonal polarized componentspassed through the photoelastic element, which are one componentpolarized in a stress direction or largest stress direction thereof andthe other component polarized in a direction perpendicular to the stressdirection thereof. The system for detecting this phase difference may bewellknown. When materials of the photoelastic element and the stressgenerating means are selected in such a way that large thermal stress isyielded in the photoelastic element in response to changing ambienttemperature, temperature detecting sensitivity can be easily enhanced.This enables the temperature measuring device of the present inventionto measure temperature over a range of from -20° C. to 70° C. withhigher accuracy, as compared with the conventional temperature measuringdevice using change of energy gap of the semiconductor depending uponambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the arrangement of an example of the temperature measuringdevice according to the present invention;

FIG. 2 is a perspective view showing a thermal expansion photoelasticcell incorporated into the temperature measuring device;

FIGS. 3 through 5 are perspective views showing variations of thethermal expansion photoelastic cell;

FIG. 6 shows the relation of the output of an amplifier relative tochanging ambient temperature when the thermal expansion photoelasticcell shown in FIG. 5 is used; and

FIGS. 7 through 11 show further variations of the thermal expansionphotoelastic cell when viewed in the direction of input light beam.

BEST MODE FOR EMBODYING THE INVENTION

The present invention will be described in detail with reference to theaccompanying drawings.

FIG. 1 shows the arrangement of an example of the temperature measuringdevice according to the present invention.

Numeral 10 represents a sensor system located at the temperaturemeasuring point and this sensor system 10 includes thermal expansionphotoelastic cell 12.

As shown in FIG. 2, thermal expansion photoelastic cell 12 comprisesphotoelastic element 14 made of a silica glass plate having length (l)of 10 mm, width (w) of 7 mm and thickness (t) of 4 mm, for example,filler members 18 and 20 embedded symmetrical and parallel relative tocenter line 16 in photoelastic element 14, and rectangular sleeve 21closely fitted onto photoelastic element 14. Filler members 18 and 20are made of epoxy resin and each of them fills a through-hole formed inphotoelastic element 14 and having a diameter of 1.5 mm, for example.The thermal expansion coefficient of epoxy resin of filler members 18and 20 is about 150 times as large as that of silica glass ofphotoelastic element 14. When the ambient temperature rises at the areawhere thermal expansion photoelastic cell 12 is located, both ofphotoelastic element 14 and filler members 18, 20 are thermallyexpanded. However, photoelastic element 14 is quite remarkably differentin thermal expansion coefficient from filler members 18 and 20.Photoelastic element 14 is forced in rectangular sleeve 21. Therefore,stress at that portion of photoelastic element 14 which is betweenfiller members 18 and 20 becomes larger in a direction of axis (x) thanin another direction of axis (y) in Cartesiain coordinates shown in FIG.2. In other words, filler members 18 and 20 work as stress generatingmeans 22 yielding stress anisotropy in photoelastic element 14 accordingto ambient temperature because filler members 18 and 20 are quiteremarkably different in thermal expansion from photoelastic element 14.

Polarizer 26 is located facing one end face 24 of photoelastic element14 when viewed in the length direction of photoelastic element 14. Thepolarizing plane of this polarizer 26 is tilted at the angle 45 degreesrelative to axis (x) of Cartesiain coordinates shown in FIG. 2. Lens 28whose optical axis is in line with axis (z) of the Cartesiaincoordinates is located outside polarizer 26. One end of optical fiber 30is located outside lens 28 while the other end thereof remote from thetemperature measuring point is optically connected to light source 34(for example an LED) through lens 32.

1/4 wave plate 38, analyzer 40 and lens 42 are located on the sameoptical axis, facing other end face 36 of photoelastic effect element14. 1/4 wave plate 38 holds one of its main axes parallel to axis (x) ofCartesiain coordinates shown in FIG. 2. The polarizing plane of analyzer40 is perpendicular to that of polarizer 26. One end of optical fiber 44is located outside lens 42 while the other end thereof remote from thetemperature measuring point is optically connected to photodetector 46which is, for example, a photodiode. Outputs of photodetector 46 areamplified by amplifier 48 and then applied to signal processor means 50,which calculates the ambient temperature at the area where sensor system10 is located as will be described later. The ambient temperature thuscalculated is displayed on display means 52. Sensor system 10 is roughlyshown in FIG. 1, but it is actually arranged that the above-mentionedcomponents are located as described above and fixed on a substrate by abonding agent, keeping only the outer faces of thermal expansionphotoelastic cell 12 exposed but the other components shielded by cover.

As already described above, photoelastic element 14 which works asthermal expansion photoelastic cell 12 is quite remarkably different inthermal expansion coefficient from filler members 18 and 20 embedded inphotoelastic element 14. When the ambient temperature rises, therefore,stress at that portion of photoelastic element 14 which is betweenfiller members 18 and 20 becomes larger in the direction of axis (x)than in the direction of axis (y). This stress corresponds to theambient temperature. When this anisotropy of stress is caused inphotoelastic element 14 made of silica glass, element 14 comes to havebirefringence or anisotropy of refractive index.

When light of a certain intensity is sent from light source 34, itenters into optical fiber 30 through lens 32 and optical fiber 30 guidesit to sensor system 10 located at the temperature measuring section. Thelight coming out of optical fiber 30 is converted by lens 28 intoparallel beams, which enter into polarizer 26. Polarizer 26 emitslinearly polarized light which includes polarized components tilted atthe angle 45 degrees relative to the direction in which the stress ofphotoelastic element 14 becomes the largest. This light passes throughphotoelastic element 14 and comes out of the other end face of theelement 14. Phase difference φ between the two orthogonal components ofthe light passing through photoelastic element 14 which are polarizedcomponent in the direction of axis (x) and polarized one in thedirection of axis (y) is yielded by the birefringence of photoelasticelement 14 which corresponds to the ambient temperature. This phasedifference φ is represented by

    φ=2πl(nx-ny)/λ                               (1)

where the refractive index of the light polarized in the direction ofaxis (x) is nx, the refractive index of light polarized in the directionof axis (y) is ny, the wavelength of light is λ, and the length ofphotoelastic element 14 in the direction in which light passes throughelement 14 is l.

Since nx and ny change according to the ambient temperature, phasedifference φ changes according to the ambient temperature, too.

The light which has passed through photoelastic element 14 then passesthrough analyzer 40, whose polarizing plane is arranged perpendicular tothat of polarizer 26, and comes to photodetector 46 through lens 42 andoptical fiber 44. Intensity (I) of the light can be expressed asfollows:

    I∞I.sub.O (1-cos2φ)2                             (2)

wherein I_(O) represents the intensity of light emitted throughpolarizer 26. When the relation of phase difference φ relative to theambient temperature is previously known, the ambient temperature can beobtained from the measured light intensity I. In the case of thisembodiment of the present invention, 1/4 wave plate 38 is interposed tomake (I) proportional to φ. Therefore, the light intensity is expressedas

    I∞I.sub.O (1+sin2φ)/2 (3)

At that range where phase difference φ is small, I is proportional to φ.

Outputs of photodetector 46 are amplified by amplifier 48 and thenapplied to signal processor means 50. Signal processor means 50calculates the ambient temperature using a previously obtainedinput/temperature calibration table and this calculated temperaturevalue is displayed on display means 52. Ambient temperature (T) can bethus monitored immediately.

Sensor system 10 for measuring temperature are composed of only opticalones in this case. Even when sensor system 10 is located in a highelectric or magnetic field, therefore, it can measure temperaturewithout being interfered with by the high electric or magnetic field. Inaddition, detection sensitivity can be easily enhanced only by selectingthe material of stress generating means 22. Because stress generatingmeans 22 can yield anisotropic stress in photoelastic element 14corresponding to the ambient temperature when the material of means 22is quite remarkably different in thermal expansion from photoelasticelement 14.

FIG. 3 shows a variation of the thermal expansion photoelastic cell.Thermal expansion photoelastic cell 60 comprises photoelastic element 62which is made of silica glass and shaped like a rectangular pole, twomembers 64 and 66 each which are made of epoxy resin and shaped like arectangular pole, and rectangular sleeve-like frame 68 made of amaterial of which thermal expansion coefficient is smaller than that ofsilica glass of photoelastic element 62, wherein two members 64 and 66which sandwich photoelastic element 62 between them are closely fittedinto frame 68.

With thermal expansion photoelastic cell 60 arranged as described above,anisotropic stress is generated in photoelastic effect element 62responsive to ambient temperature because of the difference of thermalexpansion coefficient between photoelastic element 62 and members 64,66. The thermal expansion coefficient of epoxy resin is larger than thatof silica glass. Larger stress in the direction of axis (x) can begenerated than that in the direction of axis (y) in this case inphotoelastic element 62, corresponding to changing ambient temperature.Frame 68 and members 64, 66 work as stress generating means 70 in thiscase. Thermal expansion photoelastic cell 60 can be used instead ofthermal expansion photoelastic cell 12 shown in FIG. 1.

FIG. 4 shows a further variation of the thermal expansion photoelasticcell. Thermal expansion photoelastic cell 80 comprises photoelasticelement 82 made of Pyrex glass or polycarbonate shaped a rectangularpole, outer member 84 which is made of Invar alloy and is contact at 4side faces with photoelastic element 82, and through-holes 86 and 88which is symmetrical and parallel to axis (z) in photoelastic element82.

The thermal expansion coefficient of Pyrex glass or polycarbonate islarger than that of Invar alloy. In addition, through-holes 86 and 88are made on axis (x) in this case. With this thermal expansionphotoelastic cell 80, therefore, stress in the direction of axis (y) inphotoelastic element 82 can be generated larger than that in thedirection of axis (x), corresponding to changing ambient temperature.Outer member 84 and through-holes 86, 88 work as stress generating means90 in this case.

FIG. 5 shows a still further variation of the thermal expansionphotoelastic cell. This thermal expansion photoelastic cell 100 is avariation of thermal expansion photoelastic cell 80 shown in FIG. 4. Inthe case of this variation, therefore, through-holes in photoelasticelement 82 are filled with inner members 102 and 104 which are made of amaterial with a larger thermal expansion coefficient than that of thematerial of photoelastic element 82.

With thermal expansion photoelastic cell 100, therefore, stress in thedirection of axis (x) in photoelastic effect element 82 can be generatedbecome larger than that in the direction of axis (y) responding tochanging ambient temperature, so that temperature detecting sensitivitycan be enhanced. Outer and inner members 84, 102 and 104 work as stressgenerating means 106 in this case. FIG. 6 shows the relation of ambienttemperature and the output of amplifier 48 when thermal expansionphotoelastic cell 100 is used instead of thermal expansion photoelasticcell 12 shown in FIG. 1. As apparent from FIG. 6, the output ofamplifier 48 changes almost linearly when ambient temperature changes.

FIG. 7 shows a light incident end face of thermal expansion photoelasticcell 110 which is a still further variation according to the presentinvention. Thermal expansion photoelastic cell 110 comprisesphotoelastic element 112 made of Pyrex glass or polycarbonate, outermember 114 which is made of Invar alloy and is contact at 4 side faceswith photoelastic element 112, and inner members 116, 118 which are madeof a material with a thermal expansion coefficient larger than that ofthe material of photoelastic element 112, and which are located on bothsides of element 112 in the direction of axis (x).

When thermal expansion photoelastic cell 110 is arranged as describedabove, stress in the direction of axis (x) is far larger than stress inthe direction of axis (y), in photoelastic element 112 in response tochanging ambient temperature. Temperature detecting sensitivity can bethus enhanced. Outer and inner members 114, 116 and 118 work as stressgenerating means 120 in this case.

FIG. 8 shows a light incident end face of thermal expansion photoelasticcell 130 which is a still further variation according to the presentinvention. Thermal expansion photoelastic cell 130 comprisesphotoelastic element 132 which is made of Pyrex glass or polycarbonateand is shaped as a rectangular pole, and outer member 134 which is madeof Invar alloy and is contact at 2 side faces with photoelastic element132 and leaves spaces between two sides of element 132 in the directionof axis (y).

When thermal expansion photoelastic cell 130 is arranged like this,stress in the direction of axis (x) can be far larger than stress in thedirection of axis (y), of those stresses which are generated inphotoelastic element 132 in response to changing ambient temperature.Temperature detecting sensitivity can be thus enhanced. Outer member 134and spaces 136, 138 which is left between both sides of element 132 inthe direction of axis (y) and outer member 134 work as stress generatingmeans 140 in this case.

FIG. 9 shows a light incident end face of thermal expansion photoelasticcell 150 which is a still further variation according to the presentinvention. Thermal expansion photoelastic cell 150 comprisesphotoelastic element 152 which is made of Pyrex glass or polycarbonateand is shaped a rectangular pole, outer member 154 which is made ofInvar alloy closely contact with photoelastic element 152 leaving bothend faces of element 152 uncovered and forming spaces between both sidesof element 152 in the direction of axis (y) and outer member 154, andcircular through-holes 156, 158 which is made symmetrical to axis (y)and parallel to axis (z) in photoelastic effect element 152.

When thermal expansion photoelastic cell 150 is arranged like this,stress in the direction of axis (x) can be made far larger than stressin the direction of axis (y), in photoelastic element 152 in response tochanging ambient temperature, thereby enabling temperature detectingsensitivity to be remarkably enhanced. In this arrangement, element 152extended in the direction of axis (z) and through-holes 156, 158 work asstress generating means 164. Through-holes 156 and 158 may be filledwith members made of a material with a thermal expansion coefficientlarger than that of the material of photoelastic element 152.

FIG. 10 shows a light incident end face of thermal expansionphotoelastic cell 170 which is a still further variation according tothe present invention. In this case of this thermal expansionphotoelastic cell 170, photoelastic element 172 is made of Pyrex glassor polycarbonate and shaped like a rectangular pole. Outer member 174 ismade of Invar alloy and encloses photoelastic element 172 of which bothend faces of element 172 is uncovered. A space is left between one sideof element 172 in the direction of axis (x) and outer member 174. Thisassembly of photoelastic element 172 and outer member 174 is closelyfitted into cylindrical member 176. Auxiliary member 178 is fitted intothe space between one side of element 172 in the direction of axis (x)and outer member 174 and screw 180 loads bias force to auxiliary member178 from outside cylindrical member 176 in the direction of axis (x).Circular through-holes are made symmetrical to axis (y) and parallel toaxis (z) in photoelastic effect element 172. They are filled with innermembers 182 and 184 which are made of a material with a thermalexpansion coefficient larger than that of the material of photoelasticelement 172.

When thermal expansion photoelastic cell 170 is arranged like this,cylindrical member 176, outer member 174 and inner members 182, 184 workas stress generating means 186. Therefore, stress in the direction ofaxis (x) can be made larger than stress in the direction of axis (y) inphotoelastic element 172 in response to changing ambient temperature.When screw 180 is adjusted to apply initial bias stress to photoelasticelement 172 in the direction of axis (x), the compressive stress can bekept by the initial bias stress, even if outer member 174 shrinks in acase where the thermal expansion coefficient of inner members 182 and184 is smaller than that of outer member 174.

FIG. 11 shows a light incident end face of thermal expansionphotoelastic cell 190 which is a still further variation according tothe present invention. In the case of this thermal expansionphotoelastic cell 170, column 192 is made of glass and rectangularpole-like spaces 194 and 196 are made symmetrical to axis (x) andparallel to axis (z) in column 192. That portion 198 of column 192 whichis between spaces 194 and 196 is doped with Ge or B to have a thermalexpansion coefficient different from that of the other portion thereofand to work as the photoelastic element. Therefore, the other portion ofcolumn 192 and portions 194, 196 thereof work as stress generating means200.

The thermal expansion photoelastic cell can be further modified asfollows: In the case of thermal expansion photoelastic cell 130 shown inFIG. 8, inner members made of a material with a thermal expansioncoefficient larger than that of the material of photoelastic element 132may be fitted into spaces between both sides of photoelastic element 132in the direction of axis (x) and outer member 134.

The stress generating means of the above-described thermal expansionphotoelastic cell are all intended to apply a compression force to thephotoelastic element. In the case of thermal expansion photoelastic cell80 shown in FIG. 4, however, a pull stress can be applied tophotoelastic element 82 when outer member 84 which is made of brass witha large thermal expansion coefficient is bonded to photoelastic element82 made of Pyrex glass. The stress generating means may work to applypull stress to the photoelastic element in this manner. IndustrialApplicability:

As apparent from the above, the temperature measuring device accordingto the present invention can be useful in those cases where temperaturemust be measured with high accuracy in high electric or magnetic fields.

I claim:
 1. A temperature measuring device comprising:a photoelasticelement having a center axis; a stress generating structure including:anouter member made of a material with a thermal expansion coefficientsmaller than that of the material of the photoelastic element andarranged so as to closely enclose the photoelastic element; and aplurality of inner members made of a material with a thermal expansioncoefficient larger than that of the material of the photoelastic elementand arranged in the photoelastic element or between the photoelasticmember and the outer member, the plurality of inner members beingsymmetrical to and parallel to the center axis of the photoelasticelement and causing the photoelastic element to generate anisotropicstress in response to an ambient temperature; means for providinglinearly polarized light means for providing the linearly polarizedlight along the center axis of the photoelastic element; means fordetecting a phase difference between first and second light componentsof the polarized light passed through the photoelastic element, thefirst light component being polarized in the direction of the stress andthe second light component being polarized in a different direction tothe above direction of stress; and means for converting the detectedphase difference into a temperature and displaying the temperature. 2.The temperature measuring device according to claim 1, wherein saidfirst and second light components are orthogonal light components. 3.The temperature measuring device according to claim 2, wherein saidstress generating structure includes a mechanism for applying ananisotropic bias force to the photoelastic element.